1. Field of the Invention:
This invention relates to the use of novel silver carboxylate compounds in black-and-white and color photothermographic and thermographic imaging elements.
2. Background to the Art:
Silver halide-containing, photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (a) a photosensitive material that generates silver atoms when irradiated; (b) a non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for the silver ion (e.g., that silver ion in the non-photosensitive, reducible silver source); and (d) a binder.
The photosensitive material is generally photographic silver halide that must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. For example, catalytic proximity can be accomplished by partial metathesis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); by coprecipitation of silver halide and the reducible silver source material (see, for example, U.S. Pat. No. 3,839,049); and other methods that intimately associate the photosensitive, photographic silver halide and the non-photosensitive, reducible silver source.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further processed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms (i.e., the latent image). At elevated temperatures, in the presence of the latent image, the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by a reducing agent for silver ion. This produces a black-and-white image of elemental silver.
The reducing agent for the organic silver salt, often referred to as a "developer," may be any material, preferably organic material, that can reduce silver ion to metallic silver. While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, hindered bisphenol reducing agents have traditionally been preferred.
As the visible image in black-and-white photothermographic elements is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable to reduce the cost of raw materials used in the emulsion and/or to enhance performance. For example, toning agents may be incorporated to improve the color of the silver image of the photothermographic element. Another method of increasing the maximum image density in photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials (e.g., leuco dyes) in the emulsion. Upon imaging, the dye-forming material is oxidized, and a dye and a reduced silver image are simultaneously formed in the exposed region. In this way, a dye-enhanced silver image can be produced.
A number of methods have been proposed for obtaining color images with dry silver systems. One method includes incorporating dye-forming coupler materials into the dry silver systems. Color-forming dry silver systems may include a combination of organic silver compound, a magenta, yellow, or cyan dye-forming coupler, an aminophenol developing agent, a base release agent such as guanidinium trichloroacetate, and silver bromide in poly(vinyl butyral). For example, color-forming dry silver systems may comprise a combination of silver behenate, a 2-equivalent or a 4-equivalent yellow, magenta or cyan dye-forming coupler, a sulfonamidophenol developing agent, an amine such as n-octadecylamine, and silver bromoiodide in poly(vinyl butyral).
Color images can also be formed by incorporation of dye forming or dye releasing compounds into the emulsion. Upon imaging, the dye forming or dye releasing material is oxidized and a dye and a reduced silver image are simultaneously formed in the exposed region. For example, leuco dye compounds are often incorporated into the emulsion. A leuco dye is the reduced form of a color-bearing dye. It is generally colorless or very lightly colored. Upon imaging, the leuco dye is oxidized and a dye and a reduced silver image are simultaneously formed in the exposed region.
Multicolor photothermographic imaging elements typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic elements having at least two or three distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with leuco dyes are well known in the art as represented by U.S. Pat. Nos. 3,180,731; 3,531,286; 3,761,270; 4,022,617; 4,460,681; 4,883,747; and Research Disclosure March 1989, item 29963.
Thermographic imaging constructions (i.e., heat-developable materials), imaged and processed with heat and without liquid development, are widely known in the imaging arts and rely on the use of heat to help produce an image. These elements generally comprise a support (such as paper, plastics, metals, glass, and the like) having coated thereon: (a) a thermally-sensitive, reducible silver source; (b) a reducing agent for the thermally-sensitive, reducible silver source (i.e., a developer); and (c) a binder.
In a typical thermographic construction, the image-forming layers are based on silver salts of long chain fatty acids. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. At elevated temperatures, silver behenate is reduced by a reducing agent for silver ion such as methyl gallate, hydroquinone, substituted-hydroquinones, hindered phenols, catechol, pyrogallol, ascorbic acid, ascorbic acid derivatives, and the like, whereby an image comprised of elemental silver is formed. When the reducing agent is a material that can be oxidized to form or release a dye, as for example, a leuco dye, a colored image is formed.
Many times, the thermographic construction is brought into contact with the thermal head of a thermographic recording apparatus, such as a thermal printer, thermal facsimile, and the like. In such instances, an anti-stick layer is coated on top of the imaging layer to prevent sticking of the thermographic construction to the thermal head of the apparatus utilized. The resulting thermographic construction is then heated to an elevated temperature, typically in the range of about 60.degree.-225.degree. C., resulting in the formation of an image.
The imaging arts have long recognized the fields of photothermography and thermography as being clearly distinct from that of photography. Photothermographic and thermographic elements significantly differ from conventional silver halide photographic elements which require wet-processing.
In photothermographic and thermographic imaging elements a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development and temperatures of over 100.degree. C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate temperature (e.g., 30.degree.-50.degree. C.).
In photothermographic elements only a small amount of silver halide is used to capture light and a different form of silver (e.g., silver behenate) is used to generate the image with heat. Thus, the silver halide serves as a catalyst for the development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed photographic elements use only one form of silver (e.g., silver halide) which, upon development, is converted to silver. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in a conventional wet-processed silver halide.
Photothermographic and thermographic systems employ a light-insensitive silver salt, such as silver behenate, which participates with the developer in developing the latent image. In contrast, photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic and thermographic elements is produced primarily by reduction of the light-insensitive silver source (silver behenate) while the image in photographic black-and-white elements is produced primarily by the silver halide.
In photothermographic and thermographic elements, all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic and thermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions as compared to photographic emulsions. Even in so-called instant photography, developer chemistry is physically separated from the silver halide until development is desired. Much effort has gone into the preparation and manufacture of photothermographic and thermographic elements to minimize formation of fog upon coating, storage, and post-processing aging.
Similarly, in photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).
In photothermographic and thermographic elements the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic binders such as gelatin.
Because photothermographic and thermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.) which are intended to have a direct effect upon the imaging process can vary depending on whether they have been incorporated in a photothermographic or thermographic element or incorporated in a photographic element.
Distinctions between photothermographic and photographic elements are described by D. H. Klosterboer in Imaging Processes and Materials (Neblette's Eighth Edition), J. Sturge et al. Ed, Van Nostrand Reinhold: New York, 1989, Chapter 9; and in Unconventional Imaging Processes, E. Brinckman et al., The Focal Press: London and New York, 1978, pp 74-75.
Light-sensitive recording materials suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. Such degradation occurs when a fraction of the imaging light which strikes the photosensitive layer is not absorbed, but instead passes through to the film base on which the photosensitive layer is coated. A portion of the light reaching the base may be reflected back to strike the photosensitive layer from the underside. Light thus reflected may, in some cases, contribute significantly to the total exposure of the photosensitive layer. Any particulate matter in the photosensitive element may also cause light passing through the element to be scattered. Scattered light which is reflected from the film base will, on its second passage through the photosensitive layer, cause exposure over an area adjacent to the point of intended exposure. This effect leads to image degradation. Silver-halide based photographic materials (including photothermographic materials) are prone to this form of image degradation since the photosensitive layers contain light-scattering particles (see, T. H. James, The Theory of the Photographic Process, Fourth Edition, MacMillan 1977, Chapter 20) and antihalation and acutance agents are often added to or coated with the photothermographic element. It would be desirable to have photothermographic materials with reduced light-scattering properties.
When coated onto transparent supports for viewing or projection, thermographic recording materials suffer from a phenomenon known as haze. Haze causes degradation in the quality of the viewed image and occurs when a fraction of the light which strikes the thermographic layer is not transmitted but instead is scattered. Any particulate matter in the thermographic element may cause light passing through the element to be scattered. Silver-based thermographic materials are prone to this form of image degradation since the thermally sensitive layers contain light-scattering particles such as silver salts.
Silver carboxylates which are light stable are well known to exhibit poor solubility in organic solvents at room temperature. As a result, when the silver carboxylate/organic solvent mixture is coated onto a substrate and dried, discrete particles (as opposed to a uniform distribution of molecules) of silver carboxylate are formed on the substrate. This limits their use as silver sources in photothermographic or thermographic elements requiting high transparency because, as discrete particles, such silver carboxylates cause light-scattering. The usual process of improving solubility in organic solvents by increasing the hydrocarbon chain length of the material to be dissolved provides no benefit in the case of silver carboxylates. Silver salts of carboxylic acids having a long alkyl chain (e.g., those having from 8 to 22 carbon atoms) have no better solubility than silver salts of carboxylic acids having a short alkyl chain. Accordingly, there remains a need for soluble silver carboxylate compounds for use in photothermographic and thermographic elements.
G. Smith, D. Sagatys, C. Campbell, D. Lynch, and C. Kennard, Aust. J. Chemistry 1990, 43, 1707 describe the molecular structure of a silver complex of substituted phenoxyacetates.
Japan Laid Open Patent Application JP 54-131922 (1978), discloses silver complexes of alkoxy derivatives of benzoic acid as liquid crystalline materials.
U.S. Pat. Nos. 3,330,663 and 3,554,750 disclose silver complexes of carboxylates containing thioether linkages as replacement materials for silver halide in photographic elements.
U.S. Pat. No. 4,994,352 discloses metal salts of carboxylic acids which may be substituted with alkoxy groups. These materials are incorporated with a light absorbing dye which enables the metal salt to be thermally decomposed. No specific benefits of alkoxy or polyalkoxy groups are noted.
U.S. Pat. No. 4,943,515 discloses an information recording system in which a dye complexes to a silver atom which is supplied by silver carboxylate complexes. The silver carboxylate compounds include phenoxy derivatives of acetic acid. No specific benefits of alkoxy or polyalkoxy groups are noted.